The present invention relates generally to airfoil blades (such as those used in turbines, compressors, fans, and the like in a gas turbine engine), and more particularly to such an airfoil blade having improved internal cooling.
Turbines have airfoil-shaped blades which are employed to extract useful work from a hot moving fluid. Jet engines (also called gas turbine engines) include turbines which have some turbine blades attached to a rotating turbine rotor disc and some turbine blades (sometimes called vanes) attached to a non-rotating turbine stator casing. Modern aircraft jet engines have employed internal cooling of turbine blades to keep blade temperatures within design limits. Typically, the blade is cooled by air (typically bled from the engine's compressor) passing through a longitudinally extending cylindrical internal passage, with the air entering near the blade root (the attached portion of the blade) and exiting near the blade tip (the free end portion of the blade). Known turbine blade cooling techniques include a cooling circuit consisting of series-connected longitudinally-oriented passages producing serpentine flow which increase cooling effectiveness by extending the length of the coolant flow path. Known passages of a serpentine cooling circuit include a last downstream passage containing an insert spaced-apart from passage walls and having holes for impingement cooling of such walls with all the coolant in such passage then exiting such walls through film cooling holes leading to the outside of the airfoil blade. Serpentine cooling is efficient in that it reuses the cooling air in successive longitudinal passages of the serpentine cooling circuit, and impingement cooling has high heat transfer. However, impingement cooling is wasteful in that it does not reuse the cooling air and, when it also feeds film cooling holes, its high pressure leads to separation ("blow off") of the film cooling flow from the airfoil blade outside surface. What is needed is an airfoil blade having an improved combination of serpentine flow and impingement cooling techniques.